The present invention relates to spatial light modulators, and in particular, to an electro-optic, phase-only spatial light modulator.
Spatial light modulators (SLM) have wide application in beam-steering, holographic displays, holographic memory systems, optical information processing, optical correlators and optical pattern recognition. In their most general versions, SLMs can modulate both the amplitude and the phase of an optical wavefront. However, many SLMs modulate either only the amplitude or only the phase. Although they provide less general functionality than amplitude-phase modulation, phase-only SLMs nevertheless have many important uses. For instance, they are used in phase-code multiplexed holographic memories. Phase-only modulation can also provide superior levels of discrimination in optical pattern recognition systems. More generally, the use of phase-only modulators in optical information processing systems result in higher light efficiency, since by definition phase-only masks absorb no light.
Liquid crystal SLMs are the most prevalent and are commercially available. They can provide either amplitude or phase modulation, and they are available with moderately high resolutions (xe2x89xa7512xc3x97512 pixels). Those that are based on nematic liquid crystals have switching speeds limited to 10-20 ms, while smectic liquid crystals SLMs have switching times down to the 100 xcexcs range.
An emerging technology is the micromirror array and the deformable mirror based on MEMS (micro electromechanical systems). These devices provide phase-only modulation, and, like liquid crystal SLMs, have relatively slow switching speeds.
For fast switching speeds, one must rely on electro-optic SLMs. However most designs for this type of SLM provide only amplitude modulation. Two exceptions are the SLMs based on multiple quantum wells (MQW) described in U.S. Pat. No. 5,115,335 to Soref and U.S. Pat. No. 5,488,504 to Worchesky and Ritter. However, both these devices provide only binary phase-modulation. That is, each pixel can induce only one of two possible phase-shifts in the optical wavefront.
Alternatively, H. Sato, in xe2x80x9cElectro-optic Transform Devices and Their Applicationxe2x80x9d, Proc. SPIE, 2647, p.110 (1995), describes what amounts to a one-dimensional, continuous-phase modulating SLM based on the ferroelectric material PLZT and that is programmed to function as a zoom lens. The construction is extremely simple. The device consists of a PLZT substrate that has transparent electrodes made of indium tin oxide (ITO) deposited on each side. On one side, the electrode forms a solid ground plane, while the electrode on the opposite side is segmented. Each of these electrode segments has a different voltage applied to it. This generates electrostatic fields of differing strengths between the electrode segments and the ground plane. In turn, the refractive index is shifted by differing amounts, and an optical wavefront passing through this device will experience different phase retardations at different locations.
Sato""s device consisted of only 25 electrode segments (i.e., pixels), and each electrode was connected separately to an external voltage source. If one were to extend Sato""s approach to a high-resolution two-dimensional array of pixels, then wiring each pixel to an external voltage source quickly becomes impractical. Moreover, an increasingly large portion of real estate must be devoted to providing connection leads between the electrodes and the bond pads where the external connections are made. Thus, the electrodes fill less of the SLMs aperture, and the electric fields that determine the refractive index shift are less well controlled.
An obvious solution is to integrate the drive electronics and locate each voltage source immediately behind the corresponding electrode. This way no real estate on the PLZT substrate is taken up with wiring leads to the electrode, and an almost 100% fill factor can be achieved. Unfortunately, Sato""s device required that hundreds of volts be applied to the electrodes, and it is not possible to integrate circuitry capable of such high voltage.
On the other hand, this is the approach that Worchesky and Ritter took with their MQW-SLM, which required lower driving voltages. This device has a hybrid construction. The drive electronics and MQW optical layer are fabricated on separate substrates, and then they are bonded together. The MQW layer is segmented into pixels, and each pixel must make an individual electrical connection to its drive circuit on the electronics substrate beneath. Therefore, the two substrates are indium-bump solder bonded together. For a high-resolution device, great care is required to make sure that the substrates are carefully aligned and that good contact is made for each of the many pixels. Unfortunately, this becomes progressively more difficult to do as the number of pixels increases, driving up the manufacturing cost. Moreover, the MQW layer is built up by epitaxially growing over a hundred individual layers, which also increases costs.
U.S. Pat. No. 6,535,321 to Wang and Haertling describes an SLM design that circumvents the integration problems encountered in both Worchesky""s and Sato""s design approaches. Like Sato, Wang and Haertling use PLZT. However, the required driving voltages are reduced by sandwiching the PLZT layer inside a Fabry-Perot cavity. Wang and Haertling avoid the assembly problem found in Worchesky and Ritter""s design by fabricating the integrated drive circuitry on the electronics substrate first, and then depositing the PLZT and the Fabry-Perot cavity mirrors directly on top of the drive circuitry. However, their design is capable of producing amplitude modulation only.
It is an object of the present invention to provide a fast, electro-optic, phase-only or phase-dominant spatial light modulator.
It is another object of the present invention to provide an electro-optic, phase-only spatial light modulator in which the drive electronics are integrated with the optics of the modulator so as to enable a high pixel density and a high pixel count.
It is a further object of the present invention to provide an electro-optic, phase-only spatial light modulator that is simple to manufacture and that has a lower cost of manufacture.
The present invention is an electro-optic, phase-only or phase-dominant spatial light modulator which is built around an electro-optic wafer, such as lithium niobate (LiNbO3) or lead-lanthanum-zirconate-titanate (PLZT). The electro-optic wafer used in the present invention is sandwiched between parallel conductors. The top electrode is transparent. When a voltage is applied across these conductors, an electrostatic field is generated between the conductors, and the refractive index of the wafer changes.
The spatial light modulator of the present invention also includes a totally reflecting dielectric mirror on the bottom face of the electro-optic wafer and above the bottom electrodes. Since this mirror is non-conducting, it does not interfere with electrostatic field set up between the bottom electrodes and the top ground plane. However, when light is incident from above, it passes through the transparent top electrode and the electro-optic wafer. Then it reflects off the bottom mirror, and exits out the top of the device. Because the application of voltage between electrodes changes the refractive index of wafer, the wavelength of the light inside the wafer is altered. Therefore, the phase of the light wave, at the point it exits the device, also changes.
Because the bottom electrode is segmented, a different voltage can be applied to each electrode. Thus, the refractive indexxe2x80x94and therefore the phase of the exiting light wavexe2x80x94can be manipulated to vary with position. In this way, the phase of the outgoing optical wavefront is spatially modulated.
The voltage source circuitry for each electrode is located immediately beneath that electrode. The electronics layer can also include interface logic, which, by way of example:
(a) accepts data from off-chip and changes the state of a single pixel at a time,
(b) accepts data serially from off-chip over a period of time for all pixels and then changes the states of all the pixels simultaneously, or
(c) contains various pre-set patterns of values for all pixels that can be selected in response to a command signal from off-chip.
This arrangement avoids wiring problems, and the bottom electrodes can fill almost 100% of the aperture of the device.
Depending on thickness and material of the wafer, often anywhere between several hundred and several thousand volts needs to be applied across the wafer to induce a large enough change in refractive index to cause sufficient phase retardation. However, no integrated circuit technology can sustain higher than a few hundred volts, and economical are limited to xe2x89xa6100V. In these cases, the present invention works with these more modest voltages by enhancing the effect of the resulting small xcex94n by sandwiching the electro-optic wafer inside a Fabry-Perot cavity. Accordingly, a partially reflecting dielectric mirror is deposited on the top face of the wafer. Along with the totally reflecting bottom mirror, it forms an asymmetric Fabry-Perot cavity. The resonance of the Fabry-Perot cavity works to enhance the effect of a small change in the refractive index of the SLM of the present invention. Preferably, dielectric mirrors are used because is possible to get extremely high reflectivities with such mirrors, and because, even at low reflectivities, there is negligible absorption by such mirrors.
The electronics are fabricated on their own separate wafer, typically silicon, with the top metallization layer being used for the bottom electrode pads. The mirrors and top electrode are deposited on the separate electro-optic wafer. Then, the two wafers are bonded together. Because nothing in the optics wafer needs to be segmented, no alignment is needed during bonding. Moreover, no electrical contact is needed, so the wafers can be simply cemented together, which is simple and inexpensive to do.